Conventional radio communication transmitters utilize high stability reference oscillators that require periodic adjustment to compensate for carrier drift. To meet the higher data rate constraints of future radio communication systems, e.g., future paging systems, precision automatic drift control systems will be required. Such systems will be required to constrain carrier drift with great precision. For example, some simulcast paging systems are expected to require drift control to less than 1 part per billion in the near future.
Conventional automatic drift control systems have utilized time-mark disciplined reference oscillators operating from a precision timing signal, such as a timing signal from a Global Positioning Satellite (GPS) receiver. The GPS receiver provides a 1 pulse-per-second (PPS) precision timing signal, which has been employed in conventional automatic drift control systems for measuring the frequency of the reference oscillator.
Unfortunately, the reference oscillator typically operates at a relatively low frequency, e.g., 6.25 kilohertz (KHz), compared to the carrier frequency, e.g., 900 megahertz (MHz), used for transmission by the radio communication transmitter. The relatively low frequency of the reference oscillator makes it difficult to accurately measure the frequency in a short time period, such as the 1 second period of the 1 PPS signal from the GPS receiver. This is so because there are a limited number of "events" (or cycles) that occur in 1 second of the low frequency reference signal. A 6.25 KHz signal, for example, can be measured only to an accuracy of 1 part in 6,250 without the addition of complex and expensive interpolation circuitry and/or multipliers for assessing a drift of less than 1 cycle during the measurement interval of 1 second.
Thus, what is needed is a reference oscillator automatic drift control system that can provide a much greater degree of precision over a given measurement interval than the conventional drift control system has been able to achieve. In addition, an economical automatic drift control system that provides the required precision without requiring additional interpolation circuitry and/or multipliers is needed.